Zinc acetylacetonate Zn(acac)2 is known as a catalyst for numerous reactions.
EP 0986527 B1 describes a process for reduction of carbonyl compounds with compounds bearing silanic hydrogen (SiH) and especially the reduction of esters with poly(methylhydro)siloxane in the presence of ligand-modified zinc acetylacetonate. Depending on the chosen ligand system, the reduction of methyl benzoate to benzyl alcohol is accomplished in virtually quantitative yields.
WO2011/060050 A1 concerns a coating system consisting of a binder and particles, wherein the use of particular siloxane-based modifying agents is of central importance. A route to these modifying agents is provided by dehydrogenative coupling of SiH-bearing siloxanes with polyalkylene glycol monoalkyl ethers in which zinc acetylacetonate functions as a catalyst.
WO2015/039837 A1 claims a hydroxyl-containing silicone-polyester-acrylate binder and the production and use thereof. It is elucidated therein that inter alia zinc acetylacetonate is a catalyst which in the presence of moisture promotes the hydrolysis and condensation of silyltrialkoxy groups and thus brings about the curing of the system even at room temperature. Similarly, EP 2636696 A1 too recites zinc acetylacetonate as a suitable hydrolysis and condensation catalyst for curing modified alkoxylation products comprising a non-terminal alkoxysilyl group and a plurality of urethane groups.
Zinc acetylacetonate is likewise recited as a curing catalyst, though for use in polyester-modified silicone resins, in EP 0638605 A1.
Transesterification processes on alkoxyorganosilicon compounds, run either batchwise or continuously, are typically catalysed by addition of acids or bases, as disclosed for example in U.S. Pat. No. 6,489,500 B2.
Older patent documents such as U.S. Pat. Nos. 2,917,480 and 2,834,748 recite organic acids such as monochloroacetic acid, perfluoroacetic acid or else alkaline compounds such as potassium silane oxide as catalysts to be used.
Apart from the use of pure acids or bases and devoted to the objective of providing an improved process for the transesterification of alkoxysilicone compounds, U.S. Pat. No. 3,133,111 in this connection discloses as catalyst the salt-like combination consisting of the simple aliphatic acids having 1 to 7 carbon atoms or of the chlorinated acids derived therefrom or else in particular from the perfluorinated acids derived therefrom with a basic component which comprises the alkali metal hydroxides of the alkali metals whose atomic number is greater than 11 and also ammonium hydroxide, quaternary alkylammonium hydroxides, nitrogen-containing organic bases, with the proviso that the acid represented in the salt combination is present in superstoichiometric concentration.
U.S. Pat. No. 3,801,616 concerns the production of SiOC-based liquid siloxane polyoxyalkylene block copolymers by transesterification reactions between alkoxy-comprising siloxanes and polyoxyalkylenes having at least one alcoholic function each in the presence of salt-like catalysts having a defined water solubility and a pH window defined in aqueous solution.
In the production of thermally curable silicone resins for use as electrical insulation material, U.S. Pat. No. 4,408,031 recites as transesterification catalysts titanate esters, cobalt salts of organic acids or organic acids or sulfonic acids, such as preferably para-toluenesulfonic acid or benzenesulfonic acid.
In addition to the previously mentioned alkyl titanates, for example butyl titanate, EP 1136494 A2 also recites tin compounds such as dibutyltin dilaurate.
EP 1174467 B1 is concerned with the production of heat-stable, corrosion-inhibiting polyorganosiloxane resins and as a synthetic substep provides for the reaction of the SiOR groups bonded to the resin with one or more polyhydric alcohols. Recited as suitable transesterification catalysts are for example metal catalysts based on for example magnesium, cobalt, iron, aluminium, titanium, lead, zinc or tin, for example in the form of laurates, octoates, acetates, acetylacetonates, neodecanoates or naphthalates thereof. Likewise to be employed are titanium esters or cobalt salts of organic acids or sulfonic acids, such as p-toluenesulfonic acid or benzenesulfonic acid. Recited among the suitable organotin catalysts are for example dibutyltin dilaurate, dibutyltin dioctoate or dibutyltin diacetate. Reported as particularly suitable organotitanium catalysts are for example tetra(n-butyl) titanate or tetra(isopropyl) titanate. However, in the exemplary embodiments of this document only and solely tetra(n-butyl) titanate is used for transesterification of the ethoxy functions bonded to the silicone resin with polyols, for example trimethylolpropane. The Si-bonded ethanol is liberated with modest yields of 60%. Whether the (metal) compounds deriving from the combinatorics of the recited individual compounds are in their entirety at all suitable for transesterification of alkoxy-bearing silicon compounds remains doubtful.
This doubt is supported by a comparative experiment (cf. in the examples section below, example 11) which considers the reaction of an α,ω-diethoxypolydimethylsiloxane with a polyether alcohol in the presence of zinc neodecanoate. The reaction proceeds so badly that analysis of the biphasic product may be eschewed altogether.
It must further be noted that the transesterification reaction on a highly crosslinked silicone resin is a technically low hurdle since, even in the case of an unfortunate selection of a catalyst recited in the prior art, side reactions such as undesired equilibration or skeletal rearrangement do not occur.
A much higher technical hurdle by contrast is that of reproducibly clean production of SiOC-bonded polyether siloxanes by transesterification of alkoxysiloxanes with polyetherols, particularly when the target products are inputs for very demanding applications as surface-active substances. High, if not quantitative, conversions are mandatory here in order to reliably establish the particular effect.
With the aim of catalysing the substitution reaction of alkoxy groups at the silicon, Berzate et al. investigated the reactionPhSi(OMe)3+3C7H15OH→3MeOH+PhSi(OC7H15)3 which they performed under Lewis acid catalysis, specifically also under zinc acetylacetonate catalysis (Latvijas PSR Zinatnu Akademijas Vestis, Kimijas Serija (1975), (2), 186-8). With zinc acetylacetonate the reaction comes to a halt at only 61.3% conversion even in this simple reaction system.
This sobering synthesis result obtained from a simple system characterized by the high reactivity of the employed silane body shows that the prior art would in no way contemplate the possibility of efficaciously employing zinc acetylacetonate as catalyst for the production of chemically complex systems.
This will become apparent particularly with the aid of the tests supporting the invention or of the accompanying comparative tests.
However, it has now been found that, surprisingly, zinc acetylacetonate is in fact suitable as a particularly excellent catalyst for the transesterification of alkoxysiloxanes with polyetherols and thus for the production of chemically complex systems.